The present invention relates to a method for the rapid determination of the contents of lignin, monosaccharides and organic acids in the process solutions of sulfite pulping.
The objective of the invention is to provide a method for obtaining measurement information on the contents of sulfonated dissolved lignin, monosaccharides and organic acids at short time intervals for the purpose of controlling sulfite cooking or other pulping or by-product production processes and for the characterization of the spent liquor.
The present invention is characterized in that any non-ionized compounds that can disturb the UV-absorption and other measurements are separated from the lignin material. The separation is accomplished by an ion-exclusion technique and that the measurements of contents are performed out of the liquid flow coming out from the separation column by means of the UV-method at a wavelength of 280 nm or 260 nm, by means of the refractive-index method, and/or by means of the polarimetric method.
In the paper by Shaw A. C., "Determination of Sugars in Waste Sulphite Liquor," Can. Pulp Paper Ind. 10(1957):11, 49-50, it is suggested that the ion-exclusion phenomenon should be used for the purification of the sugars of sulfite spent liquor before the determination of reducing sugars. Later, in other studies, the ion-exclusion technique has been used for lignosulfonate-sugar separations in laboratory studies (see, e.g., Felicetta C. F., Lung M., McMarthy J. L., "Spent Sulphite Liquor VII. Sugar-lignin Sulphonate Separations Using Ion Exchange Resins." TAPPI 42(1959):6, 496-502). The method has been slow, the separations have taken from an hour to several hours, and the purpose has been to perform a group separation for further analyses in view of properties and identification of the compounds in sulfite spent liquor. Ion exclusion has not been suggested in the prior art for direct determinations of contents, but it has rather been a step of preliminary treatment before different methods of determinations of contents, are performed in the laboratory manually. The most important feature of the present invention, is the elimination of compounds disturbing the UV-measurement of lignin by means of ion exclusion.
In prior art, attempts have been made to obtain the information on contents provided by the present method by direct measurements out of the process solutions. However, these measurements have been less than successful.
Traditionally, the progress of sulfite cooks has been observed visually by comparing the colour of the cooking liquor with a color standard. In the 1950's and 60's, colorimeters were introduced in the measurement of the color. This technique allowed the observation of the transparency of the solution at a certain wavelength of visible light (Meindl N., Lichtabsorptionsmessungen an Kochsauren, im Zusammenhanq mit dem Augschlussgrad und der technologischen Anwendung in der Sulfitzellstoffherstellung. Doctor's thesis, Technische Hochschule Grantz, 1961). On the basis of colorimetric measurement of the color of the cooking liquor, automation of sulfite cooking became possible. (AT Patent No. 212,687).
In the 1950's, lignin measurement based on the absorption of UV-radiation was also suggested as a basis for the cook control. In principle, UV-spectrophotometric determination of lignin can be performed at the extreme values of the UV-spectrum. Such values range from 200 to 205 nm and 280 nm to 260 nm. The "shoulder" measurement at 230 nm was also suggested. In a paper by Patterson R. F., Keays J. L., Hart J. S., Strapp R. K., Luner P., "The Spectrophotometric Determination of Lignin in Sulphite Cooking Liquor." Pulp paper mag. Can. 52(1951), 105-111, lignin measurement at 280 nm was suggested. It was, however, noticed that, during cooking, UV-absorbing compounds not derived from lignin were produced which caused error in the measurement, particularly in the final stage of the cooking. In the paper by Kleinert T. N., Joyce C. S., "Short Wavelength Ultraviolet Absorption of Lignin Substances and its Practical Application in Wood Pulping." TAPPI 40(1957):10, 813-821, measurement at 205 nm was suggested by means of very thin cuvette together with a diluting device. Later, it has been established (Schoning A. G., Johansson G., "The Ultraviolet Absorption of Sulfite Waste Cooking Liquor." Svensk papperstidn. 62(1959), 646-658, and Sjostrom E., Haglund P., "Spectrophotometric Determination of the Dissolution of Lignin During Sulfite Cooking." TAPPI 47(1964):5, 286-291 that sulfur dioxide interferes with the measurement at 205, and the measurement, consequently, does not represent lignin alone.
It is, therefore, one object of the present invention to provide a novel method to control the process solutions of sulfite pulping.
It is another object of this invention to provide a method of obtaining measurement information on the contents of sulfonated dissolved lignin, monosaccharides and organic acids at short time intervals for controlling sulfite cooking or other pulping or by product production processes.
Another object of this invention is a method for obtaining measurement information on the contents of the spent liquor.
The achievement of these and other objects will be apparent from the following description of the subject invention.